void *my_memmove(void *s1, const void *s2, size_t n)//考虑字符串重叠问题
{
	char *sc1 = s1;
	const char *sc2 = s2;
	if (sc2 < sc1&&sc1 < sc2 + n)
	{
		for (sc1 += n, sc2 += n; 0 < n; --n)
		*sc1-- = *sc2--;
	}
	else
	{
		for (; 0 < n; --n)
		*sc1++ = *sc2++;
	}
	return s1;
}
